An infrared intensity detector

ABSTRACT

An infrared intensity detector having a pair of detector elements, one shielded from and the other exposed to an incoming infrared radiation, and a thermostatted black body reference source of a characteristic temperature. The detector elements are made of a semiconductor having a remarkable temperature dependence of resistivity within a limited temperature range around the characteristic temperature. The detector offers increased detection sensitivity and finds a wide variety of applications especially where it is intended to measure an object of relatively low temperature.

United States Patent [72] Inventor Eiso Yamaka Osaka, Japan [2]] Appl.No. 886,573 [22] Filed Dec. 19, 1969 [45] Patented Nov. 9, 1971 [73]Assignee Matsushita Electric Industrial Company Limited Osaka, Japan 32Priority Dec. 25, 1968 [33] Japan [31] 44/277 [54] AN INFRARED INTENSITYDETECTOR 2 Claims, 5 Drawing Figs. [52] US. Cl 250/833 M, 73/355 R,250/83 [51] Int. Cl G011 1/16 [50] Field of Search 250/833 H, 83; 73/355Primary Examiner-James W. Lawrence Assistant Examiner-Morton J. FromeAttorney-John Lezdey ABSTRACT: An infrared intensity detector having apair of detector elements, one shielded from and the other exposed totended to measure an object of relatively low temperature.

AN INFRARED INTENSITY DETECTOR The present invention relates to animproved infrared intensity detector using a semiconductor that exhibitsabrupt variation in resistivity as a result of temperature change arounda critical temperature caused by modification of crystalline structureand, in particular, to an infrared intensity detector instrumentprovided with a thermostatted black body reference source of thecharacteristic temperature.

Conventional detector instruments utilize two detector elements havingsubstantially the same temperature-resistivity characteristics over arelatively wide range of temperature. The two elements are positionedclosely adjacent to and at a substantial spacing from each other so thatthey are placed under the same environmental conditions. With thesearrangements one of these elements is exposed to the incoming infraredradiation while the other is shielded from the radiation by means of asuitable intercepting plate. Therefore, the shielded element ismaintained at an ambient temperature to serve as a temperature referencesource. If usual thermally sensitive resistors that are commerciallyknown as thermistors are utilized for such elements, the irradiatedelement is heated by the infrared rays with the resultant temperatureincrease. The increase in temperature invites a certain reduction in theresistivity of the element. The difference in the resistance values ofthe two elements is known by the use of a suitable bridge circuit and isused to detect the emission intensity of the infrared rays. In spite ofthe reference element being used, the detector instruments of this typeare unsatisfactory in terms of detection sensitivity. Although thethermistors have a temperature dependence of resistivity over a widerange of temperature, the rate of variation in the resistivity caused bya temperature change is not high enough to provide a sufficientsensitivity. As a result, even if the detector element is subjected tothe infrared radiation of considerable intensity, the resistivity changein the element is insufiicient for certain purposes, For this reason,the requirements for performance accuracy and sensitivity are not fullymet within the conventional instruments, thereby limiting theapplications they find.

The detector instruments according to the present invention employsemiconductors that exhibit a far steeper change than usual thermistorsin resistivity around a characteristic temperature within a limitedrange. Such semiconductors are used in a pair as detector elements; onefor detecting the incoming infrared radiation and the other for keepingthe ambient temperature at the critical value. The former element isexposed to an incoming infrared radiation, as is the case ofthermistors. The intensity of the radiation is, however, detected by themeasurement of the resistivity change thereof without the use of such abridge circuit. The latter element is shielded from the infrared rays tomeasure the temperature prevailing around the two elements with highsensitivity. Therefore, if the shielded element senses the differencebetween the measured and characteristic temperatures, the difference,after amplified, may be applied to a suitable heat generator to maintainthe ambient temperature at a desired point. The detectors according tothe invention, in its preferred form, may also be provided with anelectrical furnace having a black resistive film on the inside surfacethereof. In the vicinity of the film are mounted the two detectorelements under substantially the same environmental conditions exceptthat either of the elements is shielded. With this arrangement, properlycontrolled amount of heat is evolved in the resistive film to keep theshielded element at its critical temperature. In this manner the otherelement to detect the infrared radiation is also maintained precisely atthe critical temperature.

On the other hand, the two elements are held at a close proximity to theresistive film so that the thermal resistance between the elements andfilm can be diminished to reduce the value of time constant. Thus, thetemperature of the elements is held at the desired critical value in aquick response by the heat evolved in the resistive film.

Moreover, the resistive film is made of, for example, carbon or somemetal oxides known under the trade name of Glaze Resistors so as toserve as a black body radiation source of the characteristictemperatureQTherefore, an additional black body reference source isindispensable.

It is therefore an object of the present invention to provide animproved infrared intensity detector using two pieces of a highlysensitive semiconductor as detector elements, offering increasedmeasurement sensitivity and finding wide practical applications.

It is another object of the invention to provide an improved infraredintensity detector having, in combination, a black body referenceradiation source of the characteristic temperature, whereby the detectorcan be operated with increased performance accuracy.

The present invention will be described in greater detail in conjunctionwith the accompanying drawings in which:

FIG. 1 is a typical plot of resistivity against temperature ofsemiconductors applied for the detector elements of the invention;

FIG. 2 is a sectional view of the infrared intensity detector accordingto the invention;

FIG. 3a is a plan view of the substrate plate to fabricate the detectorelements used in the detector of FIG. 2;

FIG. 3b is a section on line I-I of FIG. 3a; and

FIG. 4 is a sectional view showing a preferred example of theapplications of the detector shown in FIG. 2.

In FIG. I, the critical temperature of a certain semiconductor isrepresented by T at which the crystalline structure of the semiconductoris modified and the resistivity in the semiconductor drops abruptly. Theresistivity of the semiconductor corresponding to the temperature T isrepresented by R The semiconductors to be used in the infrared intensitydetector according to the invention may be, by way of example, vanadiumdioxide V0,, divanadium trioxide V 0 titanium monoxide TiO and silversulfide Ag S. These semiconductors are known to show steep negativetemperature characteristic in resistivity within a limited temperaturerange around T Vanadium dioxide, in particular, is known to change itsresistivity of about three digits about the critical temperature of 67C. It is, therefore, necessary to accurately maintain the temperature ofthe detector elements at the characteristic value so as to make use oftheir steep negative temperature characteristic in resistivity.

In FIG. 2, an infrared intensity detector, which is shown generally by10, comprises two detector elements 11 and 12 both of which are made ofthe same above-named semiconductive materials. The detector elements 11and 12 are mounted closely adjacent to and at a substantial spacing fromeach other on a substrate plate 13 of an insulating material such asalumina or quartz. The detector element 12 is shielded by a suitableintercepting member 14 made of a material such as metal which is opaqueto infrared radiation. A pair of terminal electrodes 15 and 16 of thedetector elements 11 and 12, respectively, are connected with a voltagesource (not shown) by way of lead wires 17 and 18, respectively.

The two detector elements 11 and 12 are maintained at the temperature T,with the resistivity of the element at the value of R This temperaturecontrol is accomplished with use of the shielded element 12 serving as atemperature detector. When the value of resistivity of the shieldedelement or temperature detector 12 differs from the reference value R,,,this difference is amplified and used to control the resistivity of theelement 12 to return to R and to restore the temperature T,,. In thismanner, the detector element 11 is also maintained at the temperature TIn operation, when infrared rays irradiate the element It in thedirection as designated by the arrow l.R. in the figure, then theelement 11 is heated with the irradiation of the infrared rays and itstemperature is augmented with the emission rate of the infraredradiation. This temperature increase causes the resistivity of theelement 11 to suddenly decrease. The decrease of resistivity is detectedby suitable electrical circuit arrangements (not shown) and isthereafter converted into radiation intensity.

FIGS. 3a and 3b show a method to fabricate the elements 11 and 12integrally with the substrate plate 13. According to a conventionalmethod, the elements 11 and 12 as utilized in the invention arefabricated by vacuum evaporation," sputtering" or printing on a planesubstrate plate with use of a mask of desired shape. Such method,however, involves the use of a complex apparatus such as a vacuum pumpand requires highly skilled techniques.

According to the method as proposed in the invention, a desired numberof cavities of an appropriate shape are formed at predeterminedpositions in the substrate plate 13. The cavities are filled with asuitable filler material for such semiconductors. The material thusfilled in the cavities is then dried or sintered with the substrateplate 13 to form the detector elements 1 l and 12. This method hasadvantages as follows:

1. The material in the cavities is secured in the cavities duringsintering process. The amount of material to be filled is, therefore,known accurately beforehand by calculating the capacity of the cavitiesso that a variation in the amount of the material can be precludedeasily.

2. Since masks made of carbon or the like need not be used,

the material is prevented from being mixed with impurities so that thepurity of the elements as required is not degraded. This is consideredimportant to keep the performance stability of the semiconductors.

Thus, the method as proposed by this invention is advantageous forfabricating the thin and small elements 11 and 12 having stablecharacteristics with required precision and reproducibility. In thismethod, moreover, the terminal electrodes l and 16 may be formed, ifpreferred, by evaporation on the opposite ends of the surfaces of thetwo elements 11 and 12.

FIG. 4 illustrates a preferred example of an infrared intensity detectorinstrument using the elements above mentioned. The detector instrumentis generally shown by 20 and comprises, as customary, the detector shownin FIG. 2, a hollow furnace element 21 having an envelope of insulatorsuch as alumina or quartz. The furnace member 21 is opened at one endand closed at the other, as shown. A black thin resistive film 22 madeof carbon or some metal oxides known commercially under the trade nameof Glaze Resistors" is coated on the inside surface of the envelope ofthe furnace member 2!. A pair of annular electrodes 23 and 24 aremounted on the upper and lower ends, respectively, of the envelope. Asuitable temperature control amplifier 25 is provided between theresistive film 22 and element 12 through the lead wire 18 for regulatingthe voltage supplied to the film 22 so as to keep the resistivity of theelement 12 at R,,. A suitable signal amplifier 26 is electricallyconnected with the element 11 by way of the lead wire 17 for detectingthe infrared rays irradiated to the element Ill. The detector element 10is shown as constructed similarly to that shown in FIG. 2 forillustrative purposes, but the detector elements 11 and 12 thereof maybe replaced with the elements fabricated in the method shown in FIGS. 30and 3b, if desired.

The whole furnace 20 is preliminarily heated by the resistive film 22and is maintained at the characteristic temperature T as previouslydiscussed. In operation, the infrared rays LR. emitted from an objectunder inspection pass through an aperture 27 formed at the open end ofthe furnace 20 and irradiate the detector element 11. Then theresistivity of the element 11 decreases abruptly due to the temperatureincrease thereof to a point suited to detect the radiation intensity bymeans of the signal amplifier 26.

This construction arrangement of the detector instruments of theinvention provides a number of advantages. Because the resistive film 22is coated on the inside surface of the furnace envelope and because thedetector elements 11 and 12 are in thermally intimate contact with thefilm 22 with the substrate plate 13 having a negligible heat capacityinterposed therebetween, the thermal time constant dictating the heatflow between the resistive film 22 and the detector elements 11 and 12can be reduced to a negligible extent. Thus, the furnace can be held inits entirely at a constant temperature T with required high accuracy. Onthe other hand, the resistive film 22 is made of, for example, coatedcarbon so that it serves as a black body reference radiation source,thereby dispensing with an additional black body reference furnace. Whatis more important, the detector elements 11 and 12 having a greattemperature dependence of resistivity, the detector instruments usingsuch detector elements offer high sensitivity to in frared radiation.

The detector instruments according to the present invention can beapplied for a wide variety of practical applications. For one thing, theinstruments can be employed advantageously for measuring an object ofhigh temperature such as a usual electrical furnace. In this instance, alarge amount of infrared radiation is emitted from the object so thatthe measurement of the temperature thereof can be conducted with utmostease. If preferred, there may be provided a suitable frustoconicalattachment at the open end of the furnace envelope so as to attenuatethe amount of the incoming infrared radiation to a reasonable value. Theinstruments of the invention will also lend themselves to detecting anobject of relatively low temperature, because of the detector elementsbeing satisfactorily sensitive to the infrared rays. This application isconsidered very important from the practical point of view, where it isintended to detect and inspect a failure of malfunction in a human bodyor to locate a failure in an integrated circuit. A portion involvingsuch failure or malfunction is usually locally heated to a highertemperature than the environments, so that infrared rays are emittedtherefrom with an appreciably higher intensity.

What is claimed is:

1. An infrared intensity detector for detecting the intensity of aninfrared radiation, comprising a hollow furnace having a resistive filmcoated on the inside surface thereof, first and Sinatra (E1565; elementsmade of aserniconductive material having a steep negative temperaturecharacteristic in resistivity within a limited temperature range arounda critical temperature, said elements being disposed in said furnace andsaid first element facing infrared radiation to be detected for de'tecting the intensity thereof and said second element being shieldedfrom the infrared radiation, and a temperature control amplifier formaintaining the temperature in said furnace at said critical temperatureconnected to said detector element and to said resistive film.

2. An infrared intensity detector according to claim 1, wherein saidsemiconductive material is a material selected from the group consistingof vanadium dioxide, divanadium trioxide, titanium monoxide and silversulfide.

1. An infrared intensity detector for detecting the intensity of aninfrared radiation, comPrising a hollow furnace having a resistive filmcoated on the inside surface thereof, first and second detector elementsmade of a semiconductive material having a steep negative temperaturecharacteristic in resistivity within a limited temperature range arounda critical temperature, said elements being disposed in said furnace andsaid first element facing infrared radiation to be detected fordetecting the intensity thereof and said second element being shieldedfrom the infrared radiation, and a temperature control amplifier formaintaining the temperature in said furnace at said critical temperatureconnected to said detector element and to said resistive film.
 2. Aninfrared intensity detector according to claim 1, wherein saidsemiconductive material is a material selected from the group consistingof vanadium dioxide, divanadium trioxide, titanium monoxide and silversulfide.